Base R is among the most powerful software packages for static data visualization, but most users are unaware of the many options for creating complex, publication-quality figures. This script lays out just a few of the many, many ways to combine datasets into complex, multipanel figures.

The data exlored here are downloaded from the World Bank data repository, which is enabled by the rWBclimate package from Scott Chamberlain and the ROpenSci organization.

First, install the packages below.

Notice that each install.packages command is commented out so that Rstudio doesn’t install them all each time the script is run. It is best to run each of those commands manually in the console, then they will be on your system each time you load them with the library command.

# install.packages('maps')
# install.packages('mapdata')
library(maps)
library(mapdata)
# install.packages('devtools')
library(devtools)
# install_github('ropensci/rWBclimate')
library(rWBclimate)

We can get some open-access data from the World Bank database with the get_historical... commands. For this example, we want data for Mexico, Guatemala, Honduras, and Costa Rica.

country.list <- c("MEX", "GTM", "HND", "CRI")
country.precip <- get_historical_precip(country.list, "year")
country.temp <- get_historical_temp(country.list, 'year')

Check out the datasets to make sure they look right.

kable(head(country.precip))
year data locator
1901 53.69402 MEX
1902 50.42450 MEX
1903 55.96359 MEX
1904 55.00967 MEX
1905 61.11870 MEX
1906 57.21757 MEX 
kable(head(country.temp))
year data locator
1901 20.52810 MEX
1902 20.80419 MEX
1903 20.17612 MEX
1904 20.64074 MEX
1905 20.43752 MEX
1906 20.48463 MEX

And then run a couple of checks to see that they are the same format and can be safely combined.

dim(country.precip)
[1] 436   3
dim(country.temp)
[1] 436   3
identical(country.precip$year, country.temp$year)
[1] TRUE
identical(country.precip$locator, country.temp$locator)
[1] TRUE

Since they are identical except for the data column, we can rearrange the columns to combine them into a single data frame.

names(country.precip)[2] <- 'precip'
country.precip$temp <- country.temp$data
head(country.precip)
  year   precip locator     temp
1 1901 53.69402     MEX 20.52810
2 1902 50.42450     MEX 20.80419
3 1903 55.96359     MEX 20.17612
4 1904 55.00967     MEX 20.64074
5 1905 61.11870     MEX 20.43752
6 1906 57.21757     MEX 20.48463
dat <- country.precip[, c('year', 'locator', 'precip', 'temp')]

Before plotting, set up a few global variables that will save lots of unnecessary code repetition later on.

temp.range <- range(dat$temp)
precip.range <- range(dat$precip)
mex <- which(dat$locator == 'MEX')
gtm <- which(dat$locator == 'GTM')
hnd <- which(dat$locator == 'HND')
cri <- which(dat$locator == 'CRI')
countries <- list(mex, gtm, hnd, cri)
countryNames <- c('Mexico', 'Guatemala', 'Honduras', 'Costa Rica')
cols <- c('darkorange', 'olivedrab4', 'cornflowerblue', 'cyan2')
yearAxis <- c(seq(1900, 2000, 20), 2009)

Separate plots

The first attempt will be to plot the lines, each in their own plot. This is useful sometimes when there is no need to plot the lines all together in the same plot. In this example, the mfrow option is set in the par command at the top. This divides the plot into 4 equal spaces, with 4 rows and 1 column. Another much more flexible way to do this would be the layout command, which is better when the individual areas should not be the same size.

par(mfrow=c(4, 1))

mars <- list(c(0, 4, 1, 4), 
             c(0, 4, 0, 4), 
             c(0, 4, 0, 4), 
             c(3, 4, 0, 4))

for(i in 1:4) {
  par(mar=mars[[i]])
  these <- countries[[i]]
  plot(dat$temp[these] ~ dat$year[these], 
       type='l', lwd=3, col=cols[i],  
       axes=FALSE, xlab='', ylab='')
  if(i %in% c(1, 3)) axis(2, las=1)
  if(i %in% c(2, 4)) axis(4, las=1)  
  text(2000, mean(dat$temp[these])-.5, 
       countryNames[i], 
       col='gray40', font=4, cex=1.3)
  points(loess.smooth(dat$year[these], dat$temp[these]), 
         type='l', lty=2, lwd=2, col='gray30')
  }
axis(side=1, labels=FALSE, at=unique(dat$year), 
     tck=-.006, col='gray50')
axis(side=1, labels=FALSE, at=yearAxis)
text(x=yearAxis-2, y=par()$usr[3]-.4, 
     yearAxis, srt=45, xpd=TRUE)

If the goal is to see that temperatures are rising across the area, this does the job, but it is pretty boring. The lines aren’t plotted together to give each context, and we have only one data type in the figure. This might be a missed opportunity for a more data-rich figure.

Plotting lines together

Since these countries are on a lattitude gradient, we can plot the lines together and see them compared to one another. This time we only create one plot, but add lines to it one at a time. There are lots of ways to automate the process without the for command, but this makes it easy to customize and easily readable.

countryNamesOffset <- c(.4, -.8, .3, .3)
plot(dat$temp ~ dat$year, 
     type='n', bty='n', axes=FALSE, ann=FALSE)
for(i in 1:length(countries)) {
  these <- countries[[i]]
  points(dat$temp[these] ~ dat$year[these], 
         type='l', lwd=3, col=cols[i])
  text(1912, dat$temp[these][9] + countryNamesOffset[i], 
       countryNames[i], font=4, col='gray40')
  points(loess.smooth(dat$year[these], dat$temp[these]), 
         type='l', lty=2, lwd=2, col='gray30')
  }

axis(side=1, labels=FALSE, at=unique(dat$year), 
     tck=-.006, col='gray50')
axis(side=1, labels=FALSE, at=yearAxis)
text(x=yearAxis-2, y=par()$usr[3]-.4, 
     yearAxis, srt=45, xpd=TRUE)
axis(2, las=1)
mtext(expression(paste('Average annual temperature (', degree, 'C)', sep='')), 
      side=2, line=2.2)

So that is more dense and gives better context for each country.

Precipitation data as polygons

Polygons are a really great way to shade backgrounds, show confidence intervals, or show depth and accumulation. So that might be a good way to visualize the precipitation data so that it stands out from the temperature data.

Each polygon is a vector of x-coordinates and y-coordinates, and they must be lined up.

  • The x-coordinates are simply the years + the same years in reverse.
  • The y-coordinates are the actual data values + a string of zeros for the bottom edge.

Here is an example with Mexico’s data.

plot(dat$precip ~ dat$year, type='n', 
     xaxs='i', yaxs='i', bty='n', 
     ylim=c(0, max(dat$precip)))
polygon(c(dat$year[mex], rev(dat$year[mex])),
        c(dat$precip[mex], rep(0, length(mex))), 
        col=cols[1], border='gray40')

To scale up this plot, we’ll plot the data one on top of another, so they have to be in a slightly different order.

  • Reorder the countries in a new sorted list, highest to lowest rainfall.
  • Also reorder the colors.
  • Make a small dataset of country means with the aggregate function. This is used to plot the average values at the far right edge.
  • Then round those values to 2 decimal points.

The next part is really important. We’ll use this code again below, so there is no reason to duplicate the whole thing. Put all of the plotting code in its own function, and then the function can be called wherever it is needed without writing lots of repatative code. This will also be done for the temperature data in the next step.

Within the function:

  • Make the empty plot.
  • Make each polygon with the for loop.
  • Make a simple axis since this plot will shrink in the final version.
  • Add the mean values to the right side.
  • Make an x-axis.

Then call the function and the plot gets made!

countriesSort <- list(cri, gtm, hnd, mex)
colSort <- c('cyan2', 'olivedrab4', 'cornflowerblue', 'darkorange')
precipMeans <- aggregate(dat$precip, by=list(dat$locator), FUN='mean')
meanLabels <- format(precipMeans$x, digits=4)

plotPrecip <- function() {
  plot(dat$precip ~ dat$year, type='n', 
       axes=FALSE, ann=FALSE, xaxs='i', yaxs='i', 
       ylim=c(0, max(dat$precip)))
  for(i in 1:4) {
    polygon(c(dat$year[countriesSort[[i]]], rev(dat$year[countriesSort[[i]]])),
            c(dat$precip[countriesSort[[i]]], rep(0, length(countriesSort[[i]]))), 
            col=colSort[i], border='gray40')
    }
  axis(2, las=1, at=c(0, 100, 200, 300))
  mtext('Annual Precipitation (cm)', side=3, line=0)
  mtext(expression(underline('mean')), side=4, at=300, 
        las=1, font=2, col='gray20', line=0, cex=.8)
  mtext(meanLabels, side=4, at=precipMeans$x, 
        las=1, font=2, col='gray20', line=0, cex=.7)
  yearSubs <- c(1901, 1960, 2009)
  axis(side=1, labels=yearSubs, at=yearSubs)
  }

par(mar=c(4, 8, 1, 9))
plotPrecip()

Temperature lines function

Here is the same function idea applied to the line plots. These will be repeated on the big final figure, so it is best to make a function with a few minor options, and then call up the one-line function wherever it is needed. This is nearly the same plot used above, but with some fancy options thrown in.

makeLines <- function(these=these, i=1) {
  plot(dat$temp[these] ~ dat$year[these], 
       type='l', lwd=3, col=cols[i],  
       axes=FALSE, xlab='', ylab='', bty='l')
  axis(2, las=1)
  text(1980, par()$usr[3], 
       countryNames[i], pos=3, 
       col='gray40', font=4, cex=1)
  points(loess.smooth(dat$year[these], dat$temp[these]), 
         type='l', lty=2, lwd=2, col='gray30')
  yearSubs <- c(1901, 1960, 2009)
  axis(side=1, labels=yearSubs, at=yearSubs)
  maxTemp <- which(dat$temp[these] == max(dat$temp[these]))
  points(dat$year[these][maxTemp], dat$temp[these][maxTemp], 
         pch=16, col='tomato', cex=1.2, xpd=TRUE)
  text(dat$year[these][maxTemp], dat$temp[these][maxTemp], 
       format(dat$temp[these][maxTemp], digits=4), 
       col='tomato', cex=.7, pos=4, font=2, xpd=TRUE)
  }

Combine maps, temp, and precip

The maps are surprisingly easy with the maps package. This time, we want the map sitting behind everything else, so the plt option in par() is the best option. The fig option would do just as well - fig includes room for margins, plt does not, so they are good for slightly different applicaitons.

  • The pdf command is commented out (as is the dev.off() command at the bottom), but it stays in place if we want to create a high-quality figure for journal submission. The eps or png functions would be fine also if that is the desired output. Since we are dealing with maps and fine lines, best to use either pdf or eps since they are vector-based graphics. Notice that the pdf command takes the same width and height measurements as the actual code chunk - They are both measured in inches.
  • Draw the world map, but use the xlim and ylim options to zoom in on our desired location.
  • Add each of our 4 countries in color.
  • Put the Carribean behind a semi-transparent rectangle so the precip plot can be put there with minimal distraction.
  • The use the plt option in par() to delineate each individual piece of the figure. It takes coordinates from 0-1, and keeps the underlying plots if new=TRUE.
  • The makeLines function is the one we created above. First for Mexico and then for the other 3 countries.
  • The last plot added is the precipitation polygon plot.
# pdf('bigMap.pdf', width=12, height=7)
map('worldHires', mar=c(.1, .1, .1, .1), 
    xlim=c(-135, -71), ylim=c(2, 35), 
    col='gray94', bg='white',  fill=TRUE)
for(i in 1:4) {
  map('worldHires', countryNames[i],
      add=TRUE, fill=TRUE, col=cols[i])
  }
rect(-86, 17, -70, 29, 
     border='transparent', 
     col=rgb(1, 1, 1, .9))
par(plt=c(0.08, .23, .57, .77), new=TRUE, 
    fg='gray20', col.axis='gray20', col.lab='gray20')
makeLines(these=mex, i=1)
mtext(expression(paste('Mean Annual Temp (', degree, 'C)', sep='')), 
      font=2, col='gray20')
par(plt=c(.1, .25, .25, .45), new=TRUE)
makeLines(these=gtm, i=2)
par(plt=c(.34, .49, .18, .38), new=TRUE)
makeLines(these=hnd, i=3)
par(plt=c(0.58, .73, .12, .33), new=TRUE)
makeLines(these=cri, i=4)
par(plt=c(0.80, .95, .5, .76), new=TRUE)
plotPrecip()

# dev.off()